1. Field of the Invention
This invention relates to color/color temperature tunable light emitting devices and in particular to solid state light sources, such as light emitting diodes, which include a wavelength converting phosphor material to generate a specific color of light.
2. Description of the Related Art
The color of light generated by a light source, in particular light emitting diodes (LEDs), is determined predominantly by the device architecture and materials selection used to generate the light. For example, many LEDs incorporate one or more phosphor materials, which are photo-luminescent materials, which absorb a portion of the radiation emitted by the LED chip/die and re-emit radiation of a different color (wavelength). This is the state of the art in the production of “white” LED light sources. The net color of light generated by such LEDs is the combined native color (wavelength) of light from the LED chip and color re-emitted by the phosphor which is fixed and determined when the LED light is fabricated.
Color switchable light sources are known which comprise red, green and blue LEDs. The color of light output from such a source can be controlled by selective activation of one or more of the different colored LEDs. For example, activation of the blue and red LEDs will generate light which appears purple in color and activation of all three LEDs produces light which appears white in color. A disadvantage of such light sources is the complexity of driver circuitry required to operate these sources.
U.S. Pat. No. 7,014,336 discloses systems and methods of generating colored light. One lighting fixture comprises an array of component illumination sources (different color LEDs) and a processor for controlling the collection of component illumination sources. The processor controls the intensity of the different color LEDs in the array to produce illumination of a selected color within a range bounded by the spectra of the individual LEDs and any filters or other spectrum-altering devices associated with the lighting fixture.
White LEDs are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught for example in U.S. Pat. No. 5,998,925, white light generating LEDs (“white LEDs”) include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorb a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor and provides light which appears to the human eye as being nearly white in color.
As is known, the correlated color temperature (CCT) of a white light source is determined by comparing its hue with a theoretical, heated black-body radiator. CCT is specified in Kelvin (K) and corresponds to the temperature of the black-body radiator which radiates the same hue of white light as the light source. The CCT of a white LED is generally determined by the phosphor composition and the quantity of phosphor incorporated in the LED.
White LEDs are often fabricated by mounting the LED chip in a metallic or ceramic cup using an adhesive and then bonding lead wires to the chip. The cup will often have a reflecting inner surface to reflect light out of the device. The phosphor material, which is in powder form, is typically mixed with a silicone binder and the phosphor mixture is then placed on top of the LED chip. A problem in fabricating white LEDs is variation of CCT and color hue between LEDs that are supposed to be nominally the same. This problem is compounded by the fact that the human eye is extremely sensitive to subtle changes in color hue especially in the “white” color range. A further problem with white LEDs is that their CCT can change over the operating lifetime of the device and such color change is particularly noticeable in lighting sources that comprise a plurality of white LEDs such as LED lighting bars.
To alleviate the problem of color variation in LEDs with phosphor wavelength conversion as is described above, in particular white LEDs, LEDs are categorized post-production using a system of “bin out” or “binning.” In binning, each LED is operated and the actual color of its emitted light measured. The LED is then categorized or binned according to the actual color of light the device generates, not based on the target CCT with which it was produced. Typically, nine or more bins (regions of color space or color bins) are used to categorize white LEDs. A disadvantage of binning is increased production costs and a low yield rate as often only two out of the nine bins are acceptable for an intended application resulting in supply chain challenges for white LED suppliers and customers.
It is predicted that white LEDs could potentially replace incandescent, fluorescent and neon light sources due to their long operating lifetimes, potentially many hundreds of thousands of hours, and their high efficiency in terms of low power consumption. Recently high brightness white LEDs have been used to replace conventional white fluorescent, mercury vapor lamps and neon lights. Like other lighting sources, the CCT of a white LED is fixed and is determined by the phosphor composition used to fabricate the LED.
U.S. Pat. No. 7,014,336 discloses systems and methods of generating high-quality white light, which is white light having a substantially continuous spectrum within the photopic response (spectral transfer function) of the human eye. Since the eye's photopic response gives a measure of the limits of what the eye can see this sets boundaries on high-quality white light having a wavelength range 400 nm (ultraviolet) to 700 nm (infrared). One system for creating white light comprises three hundred LEDs each of which has a narrow spectral width and a maximum spectral peak spanning a predetermined portion of the 400 to 700 nm wavelength range. By selectively controlling the intensity of each of the LEDs the color temperature (and also color) can be controlled. A further lighting fixture comprises nine LEDs having a spectral width of 25 nm spaced every 25 nm over the wavelength range. The powers of the LEDs can be adjusted to generate a range of color temperatures (and colors as well) by adjusting the relative intensities of the nine LEDs. It is also proposed to use fewer LEDs to generate white light, provided each LED has an increased spectral width to maintain a substantially continuous spectrum that fills the photopic response of the eye. Another lighting fixture comprises using one or more white LEDs and providing an optical high-pass filter to change the color temperature of the white light. By providing a series of interchangeable filters this enables a single light fixture to produce white light of any temperature by specifying a series of ranges for the various filters. Whilst such systems can produce high-quality white light such fixtures are too expensive for many applications due to the complexity of fabricating a plurality of discrete single color LEDs and due to the control circuitry required for operating them.
A need exists therefore for a color tunable light source that overcomes the limitations of the known sources and in particular an inexpensive solid state light source such as an LED which includes a wavelength converting phosphor material, whose color and/or CCT of light emission is at least in part tunable.